If I'm saying the following sentence, do I use "lo" or "la" for the direct object pronoun?
Mi casa es grande. Lo/la tiene tres pisos.
My house is pretty. It has three floors.
In that sentence, "it" is the subject, not the direct object. So the correct word would be "ella" (casa is femenine) but we don't say it (accademy says not to use any gender pronouns when talking about things):
Mi casa es grande. (Ella) tiene tres pisos.
The direct object pronouns "lo" and "la" are used when replacing the direct object, never the subject. In English the subject and the DO pronoun for the third neutral person is the same, it. It's easier if you use masculine or femenine third person in your examples because they change (he/him, she/her). For instance:
I've seen her. La he visto.
I killed him. Lo maté. (Here, you can use "le" because it's an acceptable case of "leísmo", but the correct form is "lo").
But it's also easy to confuse the direct with the indirect object. And there are no difference in English words as there is in Spanish (and you should try Basque, even harder). In "I told him that", the DO is "that" and "him" is the indirect. The correct translation here would be "(Yo) Le dije eso".
The easiest trick to find the DO is to ask "what" to the verb. Sometimes it's more a who than a what, but changing the phrase a little will work:
I've seen her. I've seen a flower.
What have you seen? Her (a movie, a flower...).
I killed him. I killed the wolf.
What did you kill? Him (a wolf, a snake...).